There are presently many different types of bone plate systems for securing bones, bone fragments, and/or implants in relative position so that the bones or bone fragments may fuse or heal. A shortcoming of some bone plates is the backing out or loosening of bone anchors that secure the bone plate to one or more bones. If the bone anchors loosen, the bones may not be properly secured and may move relative to each other. This may compromise the ability to achieve optimal bone fusion and bone alignment, may lead to loss of graft material, and may cause damage or loss of bone. When the bone plate is a dynamic or dynamized bone plate, such that at least some screws may move relative to the bone plate, these issues may be further compounded or exacerbated by a screw backing out.
One approach to limiting back-out of a bone anchor from a bone plate is disclosed in U.S. Pat. No. 6,599,290 to Bailey et al., which utilizes a locking ring positioned within a throughbore. The bone anchor has a head with a tapered lower segment that resiliently expands the locking ring as the bone anchor head passes through a central opening of the locking ring and into the throughbore. The locking ring resiliently retracts over the top of the bone anchor once the head is seated within the throughbore and resists back-out of the bone anchor from the throughbore. The locking ring, however, has a flange extending above the upper surface of the bone plate that may contact and irritate adjacent tissues.
United States Patent Application Publication No. 2005/0049593 to Duong et al. discloses another approach to limiting back-out of a bone anchor from a throughbore. Specifically, Duong et al. disclose an omega-shaped resilient clip that snaps into a perimetral groove on a head of the bone anchor to resist back-out of the bone anchor from the throughbore. The bone anchor head includes upper and lower radially enlarged sections that define the perimetral groove therebetween. The bone plate is relatively thick to permit both of the radially enlarged sections of the bone anchor head to be received within the throughbore. Further, the upper radially enlarged section of the bone anchor head interferes with arms of the resilient clip and limits pivoting of the bone anchor when the clip is engaged with the perimetral groove on the bone anchor head.